Attendant on the advance of plating technology, various additives have come to be added to the electrolytic solution at the time of plating for the purposes of the so-called voidless filling to restrain formation of voids in connection holes or grooves, filling up the connection holes and grooves from the bottom portions thereof (bottom-up fill), or restraining overfill, in the process of such plating as to fill up the connection holes and grooves as desired.
The additive components added to the electrolytic solution are taken into the copper film formed, and the electrolysis conditions vary at crystal grain boundaries where the concentration of the additive for promoting or restraining electrolysis is higher, making it difficult to achieve uniform electrolytic removal. Therefore, it has been difficult to uniformly form a buried wiring.
As a result, there have been the problems that local non-uniformity is generated due to the remaining of copper as the wiring material after polishing or due to over-polishing, which will lead to the generation of shortcircuit or opening of the wiring; that an unstable surface with a high surface roughness is generated; that a deficiency in the sectional area of the wiring arises from the recession of the wiring portion due to over-polishing; that a deficiency is generated in the in-plane uniformity owing to dishing; that corrosion is generated; and so on.
As for the formation of a copper film by electroplating, semiconductor makers have put their energy into development of copper plating technology since IBM announced the copper wiring technology in September, 1997. In the first stage of plating, it has been the problem to fill up connection holes and grooves in a voidless manner. Bottom-up filling is important for realizing the voidless filling, and it is said that this problem has been solved to a certain degree by control of additives. For realizing the bottom-up fill (filling up of the grooves and connection holes from the bottom portions thereof), it has been developed to use two components, i.e. a carrier and a brightener, as additives.
The carrier, basically, is a polymer having a low electric conductivity and restraining electric current so as to restrain the plating action, and has a such property that it will not easily enter into the bottom portions of the grooves and connection holes but will remain on the wafer surface. Examples of the carrier include polyethylene glycol (PGA). If the carrier is not contained in the plating solution, the plating film would be built up on the wafer surface to a height equivalent to the bottom portions of the grooves and connection holes.
The brightener, fundamentally, is ionized to enhance electric conductivity, thereby increasing the electric current and promoting the plating action. The brightener enters into the bottom portions of the grooves and connection holes, dwells in the bottom portions and constantly moves to the surface layer of copper being precipitated, thereby effecting the bottom-up fill. Even after the filling of the grooves and connection holes is completed, the concentration of the brightener is high at the areas of the grooves and connection holes, so that the precipitation of copper is faster in the areas than the other areas, resulting in the formation of swells called humps (or bumps), namely, overfill. As the brightener, there are used, for example, sulfur compounds such as MPSA (mercaptothiazole) and ammonium pyrrolidinedithiocarbamate, azo dyes, etc.
In the second stage of plating, although the voidless filling is made possible by use of the above-mentioned additives, there is the problem of overfill which would be generated under the bottom-up effect. In view of this, a three-component additive system obtained by adding the following additive to the above-mentioned two additives has come to be used.
For example, a leveler is added. Where the carrier-brightener two-component system is used, the carrier cannot suppress the promotion effect of the brightener entering into the connection holes and grooves, and, as a result, overfill occurs. The leveler, basically, has the same plating-restraining function as that of the carrier, and is deposited on the opening portions of the connection holes and grooves where the potential is concentrated. Therefore, while-preventing the formation of overhangs, the leveler traps the bottom-up fill due to the brightener at the opening portions of the connection holes and grooves, and, eventually, restrains the overfill.
However, of the additives taken into the copper film filling up the connection holes and grooves, the brightener having entered into the bottoms of via holes and having contributed to promotion of plating is left and precipitated at grain boundaries in the process of crystal growth after the bottom-up fill by crystal growth; the concentration of the brightener is higher particularly at the portions which have been grain boundary triple points or wiring grooves (depressions). For example, as shown in (a) of FIG. 4, in the case of a copper plating film 33 formed by electroplating to fill up a groove 32 formed in an insulating film 31, plating additives 41 (particularly, the brightener) will remain in a high concentration at the copper plating film portion at the groove 32. When the copper plating film 33 is then subjected to ordinary electropolishing, electric current is concentrated on the portion where the plating additives 41 remain in a high concentration, whereby the portion is dissolved selectively and precedingly, to generate a pit 51, for example, in the copper plating film 33, as shown in (b) of FIG. 4. As a result, the surface smoothness is insufficient, and it is therefore difficult to form a desired buried wiring. Though not shown, the plating additives (particularly, the brightener) would easily be concentrated also at the grain boundaries in the copper plating film, so that pits would be generated there through preceding dissolution or abnormal dissolution at the time of electropolishing, resulting in that the electropolished surface is rugged.